René I received an answer from support about this, and they told me that they convert the uploaded images to JPG, which doesn't support transparency indeed. They have no intention on changing this because PNGs would slow down the performance in the app. I don't know why that would be, but they said that I shouldn't expect this to change in the future.
They recommended me making the icons I wanted to use using the background color of the app when in dark mode. They gave me the hex colour to use, which is #222224.
If you do that, it'll look good in dark mode, but obviously not in the normal color scheme.
Good luck.